In the prior art, bicycles are usually equipped with a gear-shifting device to make the use of the bicycle more efficient.
There are bicycles equipped with a first prior art gear-shifting device comprising a series of gear wheels divided into two groups: a group associated with the shaft positioned on the axis of the pedal cranks of the bicycle and a group, which are generally more numerous, associated with the hub of the drive wheel (the rear). A mechanical transmission chain puts into mechanical contact the two groups of gear wheels.
Generally speaking, the gear wheels of the two groups are keyed parallel to each other on the hubs in such a way that a transversal movement of the transmission chain from one gear wheel to the other determines the change of the transmission ratio. The chain is generally made to translate by a user on the gear wheels of a group or simultaneously on both the groups allowing an increase and/or decrease in the speed of the bicycle, precisely because the transmission ratio between the shaft of the pedal cranks (driving axis) and the hub of the drive wheel (driven axis) is changed.
A gear-shifting device of the type mentioned above comprises at least one derailleur which is movable by means of quadrilateral linkages driven mechanically, for example by a flexible sheathed steel wire, in such a way as to move in sequence to a gear wheel selected for setting the selected transmission ratio.
In the prior art, a second type of gear-shifting device exists comprising a group of shafts divided between a primary input shaft, a secondary shaft and a tertiary output shaft and which makes it possible to have a greater degree of compactness of the transmission system relative to the derailleurs system.
As for example described in patent WO 2012/156613, a gear device of this kind has the gear wheels of the secondary shaft free to rotate relative to it except in a selection condition of a gear wheel which, temporarily and reversibly, is engaged on the secondary shaft, allowing a rigid mechanical connection between the primary shaft and the tertiary shaft.
In the second prior art type of gear-shifting device the mechanical connection between primary shaft and secondary shaft in general corresponds to a gear having a transmission ratio which is predetermined and cannot be converted as it is established during the design. The transmission ratio is therefore established with a meshing of a pair of gear wheels belonging, respectively, to the secondary shaft and to the tertiary shaft.
For this type of gear-shifting device it is known that the engaging of a gear wheel of the secondary shaft with a respective gear wheel keyed on the tertiary shaft occurs with a snap-in engaging system, comprising, for example, a slider, also called “shuttle”, which slides inside the secondary shaft which is hollow inside.
The slider is positioned at a gear wheel, sliding inside the secondary shaft and allows the engaging on the respective shaft keeping mechanically connected the primary shaft with the tertiary shaft, precisely through the secondary shaft.
In this way, as well as having established a mechanical connection between the input and output (respectively, between the pedal cranks of the bicycle and the pinion of the transmission chain), a predetermined and defined transmission ratio has been established by the user by engaging the selected gear wheel on the respective secondary shaft.
The change from one transmission ratio to a new one occurs by sliding the slider inside the cavity of the secondary shaft to the new predetermined position, and therefore to the new transmission ratio.
The slider or shuttle is moved inside the cavity of the shaft by using mechanical type transfer means, of a more or less elaborate and sophisticated nature.
In a different prior art solution, for example in patent WO 2008/142219, the gear-shifting device always comprises three shafts interacting with each other, of which at least one is coaxial with a second shaft, in particular the primary shaft is coaxial with the tertiary shaft and they are independent during the rotation by interposing rolling bearings. The shaft secondary is parallel and positioned in the proximity of the first two shafts. This variant embodiment of the gear-shifting device comprises, similarly to what is described above, the operation of a “shuttle” slider which, sliding inside a cavity made in the secondary shaft, allows the selection of a gear wheel which, after engaging on the shaft, allows the rotational movement between the primary shaft (input) and the tertiary shaft (output) by means of a suitable transmission ratio.
According to the prior art, the gear-shifting devices for bicycles have some drawbacks which limit the use both in performance and sector application terms.
In the case of a gear-shifting device of the derailleur type, the system comprises quadrilateral linkages which require a precise setting up and a programmed adjustment and maintenance after a certain time of use and/or mileage of the bicycle.
Depending on the amount of use (intense or normal), the derailleur gear-shifting device can have slight problems (increasingly long gear-shift times, gear shifts less precise), until more serious problems occur of excessive wear or failure of certain more highly stressed mechanical parts. Sometimes, the transmission chain breaks.
Moreover, the times for changing the ratio of a derailleur gear-shifting device are rather to long for a good use of the bicycle.
In effect, the gear shift times of the derailleur gear-shifting device constitute an important limitation which often impacts on the comfort of the user who is forced to reduce the force transmitted during pedalling, therefore accompanying the rotation of the transmission chain to favour the engaging of the new transmission ratio.
During this step, which lasts a few seconds, the user is forced to pedal without delivering the force which would normally be exerted to move forward at the predetermined speed, thus losing speed and increasing the physical effort due to the subsequent recapturing step.
Another drawback regarding this type of gear-shifting device is the exposure of its components to atmospheric conditions, such as rain, dust, mud and detritus, which can obstruct and/or block the movement between some parts of the gear-shifting device.
Often, during a situation of very harsh and continuous use of the bicycle, the gear-shifting device described above may suffer from nearly all the above-mentioned drawbacks.
The second prior art type of gear-shifting device also suffers from several problems especially in terms of gear shift times which are incompatible with an intensive use of the gear-shifting device and the bicycle.
This drawback can put the user in difficulty as he/she is forced to slow down considerably, immediately change gear and then recover the distance lost with greater effort and force.
In general, a gear-shifting device of this kind is mechanically more delicate; in effect the device in question is not free of failures and breakages especially of the system for driving the “shuttle” slider, an event which would completely put the gear-shifting device out of use as it would remain locked in a single transmission ratio of the bicycle or in an intermediate position between two selection positions which would not allow the engaging of any transmission ratio.
In addition, the gear-shifting device may be bulky and heavy.